The present invention relates generally to navigation systems and more particularly to a navigation system which changes its operational mode based upon a figure of merit determination for its current position solution.
Known vehicle navigation systems include a plurality of sensors which determine the position of the vehicle relative to a map database of roads. The navigation system also generally includes a user interface, such that the user can select a destination relative to the database of roads. The navigation system calculates a recommended route from the current position to the selected destination and communicates the recommended route to the driver with turn-by-turn instructions.
As the driver guides the vehicle along the recommended route toward the destination, the vehicle navigation system continuously calculates the position of the vehicle relative to the map database. The vehicle navigation system displays the current position of the vehicle on a map display. Turn-by-turn instructions to the driver are generated based upon the current position of the vehicle relative to the route, i.e., a predetermined distance from the next turn. If the driver drives the vehicle off of the recommended route, the vehicle navigation system automatically recalculates a new route from the current position of the vehicle to the destination.
The accuracy of the position of the vehicle relative to the map database as determined by the vehicle navigation system fluctuates. As is well known, a position solution from a GPS receiver may be in error up to 100 meters. Further, if GPS signals are temporarily unavailable, the error of the vehicle navigation system position solution relative to the map database could be even larger.
This presents several problems for the vehicle navigation system. First, if the vehicle navigation system erroneously determines that the vehicle is off the recommended route, the navigation system will recalculate a new route from the erroneous new current position of the vehicle. Further, turn-by-turn instructions to the driver will be generated at the wrong time, or the wrong instructions will be generated, if the navigation system calculates an erroneous position on the recommended route.
The navigation system of the present invention changes its operational mode based upon a figure of merit determination. The figure of merit determination is a quantification of the level of confidence or level of certainty of the current position solution of the navigation system.
If the figure of merit exceeds a predetermined threshold, the navigation system operates normally. The navigation system treats the current position solution as accurate and, based upon the current position solution, generates turn-by-turn instructions and automatically recalculates new routes as necessary.
If the figure of merit drops below a first predetermined threshold while in route guidance mode, the navigation system will automatically disable the automatic route recalculation. Then, if the current position solution indicates that the vehicle has deviated from the recommended route, the navigation system will not automatically recalculate the new route from the current position solution to the destination. Rather, the navigation system will display a map with the recommended route highlighted and the vehicle on the map at the current position solution. The user can manually indicate to the navigation system that a new route should be calculated. Since the figure of merit is below a first predetermined threshold, the level of certainty that the vehicle has actually deviated from the recommended route is insufficient to warrant recalculating the route.
If the figure of merit drops below a second predetermined threshold while the vehicle is in a route guidance mode, the vehicle navigation system enters a second operational mode. In the second operational mode, a list of the remaining turn-by-turn instructions to the destination along the recommended route is displayed to the driver. In this second operational mode, although the navigational system does not provide a current vehicle position solution, the driver can follow the turn-by-turn instructions on the display until a position solution which exceeds the second predetermined threshold is obtained.
If the figure of merit drops below a third predetermined threshold, the navigation system enters a third operational mode in which a map of the surrounding area surrounding the last position solution is displayed. Thus, although the navigation system is temporarily unable to indicate the present position of the vehicle relative to the map database, the navigation system displays a map of the surrounding area for the user to view. The user can use the map to navigate the vehicle until a position solution having a figure of merit exceeding the third predetermined threshold is obtained. The vehicle navigation system will switch to the third operational mode if the figure of merit drops below the third predetermined threshold while the vehicle navigation system is not in a route guidance mode, i.e., the user has not selected a destination and there is no recommended route or turn-by-turn instructions.